1. Field of the Invention
The present invention relates to a sterilizer and a sterilization method which utilize a high voltage. More particularly, the present invention relates to a sterilizer utilizing a high voltage which can be used for solid products in general, e.g., foods, drugs, Chinese herb medicines, cosmetics, feedstuffs, and fertilizers, and in particular, which can be optimally used for foods unsuitable for being subjected to heating; and a sterilization method utilizing such a sterilizer.
2. Detailed Description of the Prior Art
Conventionally, heat sterilization has been generally employed as a method for sterilizing foods. Heat sterilization, however, causes changes in components, aromas, flavors, colors, etc., the escaping of active ingredients, and the like, thereby deteriorating the qualities of foods.
A method for sterilizing a food by bringing the food into contact with heated water vapor (Japanese Patent Publication No. 63-50984) has been suggested. Where this method is employed, however, changes and deterioration in a food quality are likely to occur because of the loss of an aroma component, the penetration of a water vapor into the food, and the like.
Methyl bromide fumigation has been conducted in order to sterilize rice and kill insects in rice. However, methyl bromide is designated as a dangerous substance due to its high inflammability. Methyl bromide is also designated as a deleterious substance, and a blister is formed when one""s skin touches methyl bromide. Therefore, this method lucks in safety. In addition, when methyl bromide is released into the atmosphere and reaches the ozone layer around the globe, methyl bromide depletes the ozone layer due to its high potential to deplete the ozone layer.
Although ozone has been used for sterilization, the obtained sterilization effect is not sufficient. Also, in order to suppress an increase in the number of molds, low temperature storage (e.g., 15xc2x0 C., 70-75% RH) has been conducted, long-term low temperature storage may cause an increase in the number of molds.
Radiation sterilization which utilizes cobalt-60 or the like is permitted to be used for adzuki bean, corn, spices, and the like. However, such a method requires a large equipment, and thus the equipment cost is high. In addition, shielding is required in order to prevent radiation from leaking to the outside.
If the number of microorganisms, in particular, the number of molds is increased during the storage of unpolished rice, for example, in the worst case scenario, the commercial value of the unpolished rice is lost, and the rice is no longer suitable for eating. It cannot be expected that spore forming bacteria attached to the rice will become extinct even at the time of cooking the rice.
Some foods are not suitable for being subjected to heat sterilization. For example, rice which is a staple food for Japanese people is not suitable for being subjected to heat sterilization or chemical treatments.
The first object of the present invention is to provide a sterilizer: which can easily and efficiently sterilize various kinds of bacteria, including spore forming bacteria attached to rice, at a room temperature and an atmospheric pressure so as to reduce the number of bacteria, without deteriorating qualities of a flavor, a taste, a beneficial effect, a power, dispersibility, and the like, which are inherent to an object to be sterilized (hereinafter, referred to as xe2x80x9csterilization objectxe2x80x9d) such as a food, a drug, a Chinese herb medicine, a cosmetic, a feedstuff, and a fertilizer; which does not require pretreatment and/or after-treatment; which is highly safe; and which does not cause the ozone layer depletion problem The second object of the present invention is to provide a sterilization method which can easily, economically and efficiently sterilize various kinds of bacteria, including spore forming bacteria attached to rice, at a room temperature and an atmospheric pressure so as to reduce the number of bacteria by using the above-described sterilizer, without deteriorating qualities of a flavor, a taste, a beneficial effect, a power, dispersibility, and the like, which are inherent to a sterilization object such as a food, a drug, a Chinese herb medicine, a cosmetic, a feedstuff, and a fertilizer.
The inventors of the present invention conducted in-depth studies in order to solve the conventional problems. Consequently, they found that various kinds of bacteria including spore forming bacteria can be sterilized easily, efficiently, and economically by interposing a sterilization object between a discharge side electrode and a ground side electrode to which a high AC voltage or a pulsed high voltage is applied and by causing a pulse streamer discharge between the above-described electrodes at a room temperature and an atmospheric pressure in the atmosphere controlled to have a humidity suitable for sterilization, thereby completing the present invention.
In order to solve the above-described problems, the present invention according to claim 1 is a sterilizer which comprises: a power supply section for generating a high voltage; a processing device having a discharge side electrode and a ground side electrode to which the generated high voltage is applied; and humidity control means for controlling a humidity in the processing device. A sterilization object is interposed between the electrodes of the processing device, and a pulse streamer discharge occurs between the electrodes.
According to the present invention, various kinds of bacteria are sterilized by causing a pulse streamer discharge between the above-described electrodes.
While a corona discharge during the application of a DC voltage causes only the vicinity of the discharge side electrode to emit light, a pulse streamer discharge can excite a much wider area into a plasma state. The reasons for this are as follows. First, a pulse streamer discharge can instantaneously apply a voltage higher than a spark voltage in the case of a DC corona discharge, thereby broadening the high electric field region. Second, while a discharge is suppressed in the case of a DC corona discharge because of a reduced electric field strength in an ionization region near the discharge side electrode due to ionic space charge, discharge suppression due to a space charge electric field is very small in the case of a pulse streamer discharge because of the rapid start-up of the voltage.
The present invention according to claim 2 is characterized in that the discharge side electrode comprises a large number of needle-shaped electrodes which are provided with a predetermined distance apart from one another in the sterilizer according to claim 1.
If plate-shaped electrodes are respectively used as the discharge side electrode and the ground side electrode, the surface areas thereof are increased. Thus, there is a need to increase the capacity of a power supply section for generating a high voltage. Also, both of the plate-shaped electrodes need to be absolutely parallel to each other. If the parallel state is not maintained, a discharge occurs only in a certain area. In the case where a needle-shaped electrode is used as the discharge side electrode and a plate-shaped electrode is used as the ground side electrode, wide-ranging sterilization of sterilization objects interposed between the aforementioned electrodes cannot be achieved, resulting in uneven sterilization. If a large number of needle-shaped electrodes which are disposed with a predetermined distance apart from one another are used, a uniform discharge occurs and wade-ranging and uniform sterilization of sterilization objects interposed between the electrodes can be thereby achieved. Thus, uneven sterilization does not occur.
In order to prevent rusting and to provide durability, the needle-shaped electrode may be preferably made of a material such as a stainless material, a platinum material, or the like. Preferably, the tip of the stainless needle-shaped electrode may have a thickness of about 0.1 to 1 mm Ø. More preferably, the tip of the stainless needle-shaped electrode has a thickness as thin as about 0.1 to 0.5 mm Ø.
The distance between the needle-shaped electrodes is not limited to any particular value since the applied voltage, pulse number, input energy (cal/cm3), electric field strength, and the like are varied depending on the kind and form of a sterilization object, the kind and form of a bacterium, and the like. It is preferred that the distance between the needle-shaped electrodes is suitably determined so that a sterilization object can be sufficiently and uniformly sterilized without deteriorating the qualities of a flavor, a taste, and the like, which are inherent thereto.
However, it is desirable that the aforementioned distance is selected generally from the range of 5 mm to less than 80 mm, preferably from the range of 10 mm to 60 mm, and most preferably from the range of 25 mm to 30 mm. If the distance is less than 5 mm, it is possible to sterilize unpolished rice. However, the capacity of a power supply which will be described later needs to be increased, and thus, such a distance selection is uneconomical. If the distance is 80 mm or greater, wide-ranging and uniform sterilization of sterilization objects which are interposed between the above-described electrodes cannot be achieved, resulting in uneven sterilization.
The present invention according to claim 3 is characterized in that the ground side electrode comprises a plate-shaped electrode whose entire surface is covered with an insulating plate in the sterilizer according to claim 1 or 2.
It is preferred to dispose an insulating plate on a surface of the ground side electrode opposing to the discharge side electrode so as to cover the entire surface thereof so that, in a pulse streamer discharge, a discharge from the needle-shaped electrode is not shorted to the junction. By disposing the insulating plate, a discharge occurs safely and uniformly, thereby making it possible to sterilize a sterilization object uniformly and efficiently.
The present invention according to claim 4 is characterized in that the insulating plate is a ceramic plate in the sterilizer according to claim 3.
An acrylic plate, a glass plate, or the like is generally employed as an insulating plate used for the above-described purpose due to its low price. However, an acrylic plate, a glass plate, or the like lacks in durability and strength. Therefore, when an applied voltage is increased, it is preferred to use a ceramic plate having high durability, strength, and insulating property. The ceramic plate may be a large single sheet of plate, or a 1xc3x971 cm small thin plate with the thickness of 1 mm, for example. Small ceramic plates are cheap because they are mass-produced, and it is possible to use many small ceramic plates as a large ceramic plate by binding them with one another.
The present invention according to claim 5 is characterized in that the ceramic plate is a ceramic plate having a high withstand voltage in the sterilizer according to claim 4.
Specifically, an alumina plate, for example, may be preferably used as a ceramic plate with a high withstand voltage, and more preferably, a high purity alumina plate can be used.
The present invention according to claim 6 is characterized in that the processing device is made of a material having a volume resistivity in the range of 1010 to 1014xcexa9xc2x7cm or more in the sterilizer according to any one of claims 1 to 5.
In order to sterilize a sterilization object safely, a processing device for sterilizing a sterilization object may be preferably made of a highly insulating material having a volume resistivity in the range of 1010xcx9c1014xcexa9xc2x7cm or more. Specifically, such a material may be an electrical insulating material such as a polyolefin resin, an acrylic resin, a polycarbonate resin, various engineering plastics, or a FRP.
According to the present invention of claim 7, in a processing device having a discharge side electrode and a ground side electrode to which a high voltage generated in a power supply section for generating a high voltage is to be applied, a sterilization method comprises the steps of: interposing a sterilization object between the electrodes to which a high voltage is applied, and causing a pulse streamer discharge between the electrodes at a room temperature and an atmospheric pressure and in the atmosphere in which a humidity is controlled to be suitable for sterilization, thereby performing sterilization.
Various kinds of bacteria including spore forming bacteria can be sterilized easily, efficiently, and economically by interposing a sterilization object between the discharge side electrode and the ground side electrode and by causing a pulse streamer discharge between the above-described electrodes at a room temperature and an atmospheric pressure and in the atmosphere in which a humidity is controlled to be suitable for sterilization.
The humidity suitable for sterilization is not limited to any particular value since it varies depending on the kind and form of a sterilization object, the kind and form of a bacterium, and the like. It is preferable that the humidity is suitably determined so that a sterilization object can be sufficiently and uniformly sterilized without deteriorating the qualities of a flavor, a taste, and the like, which are inherent thereto.
The present invention according to claim 8 is characterized in that a pulsed high voltage of a positive polarity is applied to the discharge side electrode in the sterilization method according to claim 7.
If a pulsed high voltage of a negative polarity is applied to the discharge side electrode instead of applying a pulsed high voltage of a positive polarity thereto, dirt, dust, and the like may be attached to the discharge side electrode, possibly failing to obtain a uniform and stable discharge. Thus, applying a pulsed high voltage of a negative polarity to the discharge side electrode is not preferable.
The present invention according to claim 9 is characterized in that a pulsed high voltage with a start-up time of 10 nanoseconds or more and a duration of 1 microsecond or less is applied to the discharge side electrode in the sterilization method according to claim 7 or 8.
If a pulsed high voltage with a start-up time on the order of 10 nS (nanoseconds) or more and a duration of about 1 xcexcS (microsecond) or less is applied to the discharge side electrode, a streamer discharge linearly extending from the discharge side electrode occurs. Thus, a wide area between the electrodes can be exited into a plasma state, and a sterilization object interposed between the electrodes can be uniformly sterilized.
The thus-structured sterilizer of the present invention has a simple structure, and can easily and efficiently sterilize a sterilization object at a room temperature and an atmospheric pressure and in the atmosphere in which a humidity is controlled without deteriorating the qualities inherent to the sterilization object. In addition, this sterilizer requires no pretreatment and/or after-treatment, is highly safe, and does not cause the ozone layer depletion problem Although a high voltage is used, the current value is very small. Therefore, this sterilizer is safe and requires a low energy. Since no chemicals are used and no heating is performed, changes hardly occur in the qualities of a sterilization object, and no chemicals or the like stay behind. Since the temperature of a sterilization object rarely rises, it is possible to sterilize a sterilization object which is not suitable for being subjected to heating.
According to the above-described sterilization method of the present invention, it is possible to easily, economically, and efficiently sterilize various kinds of bacteria, including spore forming bacteria attached to rice, at a room temperature and an atmospheric pressure and in the atmosphere in which a humidity is controlled so as to reduce the number of bacteria, without deteriorating qualities of a flavor, a taste, a beneficial effect, a power, dispersibility, and the like, which are inherent to a sterilization object such as a food, a drug, a Chinese herb medicine, a cosmetic, a feedstuff, and a fertilizer.